A spectrometer is used to analyze a beam of light into its component colors. FIG. 1 illustrates a simple prism spectrometer 10. Light from a luminous source I passes through a slit S and is collimated by lens C before impinging on the prism P. The index of refraction of the prism P varies as a function of wavelength. This is called dispersion. The index of refraction is greater for shorter wavelengths. Therefore, by Snell's law, the prism bends the violet light into a larger angle than the red light. Lens L.sub.1 focuses the light and forms a real image of the slit S at image plane A. The real image is called the spectrum of the luminous source I. The width W of the real image is a function of the maximum-minimum wavelength range of the luminous source I, the dispersion of prism P, and the focal length of lens L.sub.1.
As shown in FIG. 1, the lateral location of the image of the slit S in the image plane A is a function of the wavelength. If the light from luminous source I is monochromatic, i.e. a single wavelength (color), then only one image of the slit is formed. The color of the slit image is the color of the monochromatic light. If the light is polychromatic, i.e. white, then a continuum of slit images is formed, each at a slightly different location, and each at a slightly different color. In the example shown in FIG. 1, for example, the solid rays represent red light, and the dashed rays represent violet light.
It is well known that a thin straight slit is preferable to avoid overlapping of the images. If the slit is made wider, the purity of the slit images is reduced. In other words, at any point in image plane A, the image becomes a blend of slightly different wavelengths from the left, center and right side of the slit. This blending is the math function called convolution.
One application in which spectrometers are used is in obtaining aerial reconnaissance imagery, wherein the spectrometer is located on an aircraft, and a detector array is located at the image plane. Multispectral imagery is very low intensity because the spectrometer spreads the light received from objects on the ground across many detector elements.